Embedding "for" structures allows repetition in two dimensions.
Original Processing.org Example: EmbeddedIteration
// All Examples Written by Casey Reas and Ben Fry
// unless otherwise stated.
float box_size = 11;
float box_space = 12;
int margin = 7;
size(200, 200);
background(0);
noStroke();
// Draw gray boxes
for(int i = margin; i < width-margin; i += box_space) {
for(int j = margin; j < height-margin; j += box_space) {
fill(255 - box_size*10);
rect(j, i, box_size, box_size);
}
box_size = box_size - 0.6;
}